Iron-doped cobalt phosphide nanowires prepared via one-step solvothermal phosphidization of metal–organic frameworks for the oxygen evolution reactions

Abstract

Efficient and stable oxygen evolution reaction (OER) electrocatalysts are essential for improving the overall efficiency of water electrolyzers. Iron-doped cobalt phosphide (CoFeP) nanowires are prepared via a one-step solvothermal process using a cobalt metal–organic framework (Co MOF), P₄ and dissolved Fe²⁺. The ion-exchange between Fe²⁺ and the Co MOF occurs first, and then phosphides are formed. CoFeP exhibits the Co₂P phase, and there exists an electronic interaction between Co and Fe. When supported on nickel foam (NF) to catalyze the OER in alkaline solutions, CoFeP/NF exhibits an overpotential of 240 mV at a 10 mA cm⁻² OER current density, which is much lower than that of Co₂P/NF. Kinetic analyses indicate that the lattice oxygen oxidation mechanism occurs at the CoFeP/NF surface, whereas the adsorbate evolution mechanism dominates at the Co₂P/NF surface. The change in the mechanistic pathway is due to the increased acidity of the Co⁴⁺ site in CoFeP, which favors the decoupled proton–electron transfer process. Lower apparent activation energy, charge transfer resistance and Tafel slope values, and higher electron rate constants are observed on the CoFeP/NF surface, and the charge redistribution induced by Fe during the OER process alters the adsorption energy of the hydroxyl groups at the Co sites, leading to more facile OER kinetics, as proved by density functional theory calculations. CoFeP/NF shows good long-term durability in alkaline solutions, and the conversion of surface phosphides to oxides and (oxy)hydroxides is observed.